This invention relates to an exhaust gas recirculation (EGR) system that reduces thermal nitrogen oxides (NOx). More particularly, this invention relates to an EGR system that reduces thermal NOx by varying a level of EGR based on a Wobbe index and/or reactivity of incoming fuel, a turbomachine system having the EGR system and an EGR control method.
EGR can be used to alter and control the production of thermal NOx. In a typical EGR system, exhaust gas is recirculated from a turbine towards an air inlet portion of the system. The quantity of exhaust gas to be recirculated may increase as a load of a generator of the system lowers. Therefore, it is necessary to regulate the amount of exhaust gas to be recirculated, to thereby enable the temperature of a combustor of the system to remain constant.
Several methods for controlling thermally generated NOx in EGR systems have been used. One method uses a diluent such as water or steam to reduce the flame temperature and limit NOx. Another method controls the EGR flow rate based on turbine loading information, to thereby minimize NOx emissions. In the latter disclosed method, the amount of exhaust gas recirculated to the compressor may be controlled based on the load of the generator, the rate of the flow of fuel to the combustor and the load demand of an external system powered by the generator, for example.
There are a number of different types of fuels that can be used in a turbomachine system. Each fuel type has a different Wobbe Index and reactivity. Therefore, the use of different types of fuels in the turbomachine system may change the temperature of the combustion process. There is a need for an EGR system that can vary the amount of EGR based on the Wobbe Index and/or reactivity of incoming fuel to the turbomachine system, to thereby permit continuous turbomachine operation during times of fluctuation in the type of fuel being used.